This invention relates to improvements in printed circuits and more particularly concerns an improved process of manufacturing printed circuit boards.
Printed circuit boards are widely used and several different methods have been proposed for manufacturing the boards. A technique used in many of these manufacturing processes involves the screening of plating resist onto a base panel to define circuit traces and areas on the panel at which connection pads and through holes are to be formed. The resist may be deposited on the panel in both negative and positive formation depending upon the sequence of steps being followed. The present invention involves the use of a novel sequence of resist-applying, metal-depositing, and resist-removing steps which results in a printed circuit in which the amount of metal deposited on the panel to form and protect the circuitry is minimized. Further, the improved method facilitates accurate inspection of the circuit during the manufacturing process and makes it possible to inspect the circuitry in a manner that is efficient from both a time-consuming and an accuracy viewpoint.
Some of the above-mentioned prior art techniques are disclosed in the following patents: U.S. Pat. No. 3,702,284 to Merkenschlager; U.S. Pat. No. 3,742,597 to Davis; U.S. Pat. No. 3,772,101 to Chambres et al.; and U.S. Pat. No. 4,101,111 to Mack. However, the method of the present invention involves a sequence of steps which is not disclosed in these patents and which features the formation of a positive image of the desired circuit in resist at an early stage of the process to make circuit inspection and touch-up an efficient and accurate operation. In addition, after the circuit is defined, no extra metal is deposited on the circuit during the entire process, and thus added plating steps, which can result in defective panels and the reworking of the circuits, are avoided. Other and further features and advantages of the method will be apparent from the following description of a preferred embodiment of the invention.